A radome is a structure that is used to shelter and protect an underlying antenna from a surrounding environment. A radome may be used, for example, to protect an antenna from high winds, icing, and/or temperature extremes in an area surrounding the antenna. Radomes generally comprise a rigid or semi-rigid structure that partially or fully envelopes the antenna and are thus, at least partially, within the signal flow path of the antenna. For this reason, radomes are normally designed to have relatively low transmission loss (i.e., to be transparent) within the operational frequency range of the antenna. If a radome is to be used in connection with a multi-frequency antenna (i.e., an antenna operative in two or more distinct frequency bands) , then the radome should be transparent in multiple frequency bands. As can be appreciated, design of such multi-frequency radomes can be difficult.
One type of radome structure, known as a material-tuned randome, utilizes one or more layers of dielectric material to achieve a desired frequency response. That is, one or more dielectric layers, each having a predetermined thickness and dielectric constant, are stacked in a manner that synthesizes a desired frequency response. Design techniques for achieving a material-tuned radome having a relatively low loss "passband" within the operational frequency range of an antenna are well known in the art. In addition, material-tuned radome design techniques for achieving multiple passbands for use in connection with, for example, multi-frequency antenna systems are also known. Multi-frequency material-tuned radomes are relatively complex structures that normally include a large number of dielectric layers. To achieve a desired frequency response, the thickness of the various dielectric layers of the multi-frequency radome (deposited during radome fabrication) must be relatively precise. At higher frequencies, however, dimensional control of these layers becomes difficult, thus complicating the multi-frequency radome fabrication process.
Even greater difficulty is encountered when it is necessary to add a new, higher frequency passband to an already existing material-tuned radome design. This may be necessary, for example, if a new antenna that is operative in a different frequency range is being added to a corresponding antenna system. If the existing radome is not transparent in the new frequency band, then the radome must either be modified to add a new passband or the radome must be replaced with a new multi-frequency design. As can be appreciated, it is preferable that the old radome be modified to avoid the costs associated with the design and development of a new radome. However, such modifications can be complicated and are sometimes just as costly as a redesign. Therefore, there is a need for a multi-frequency radome structure that is relatively simple and inexpensive to design and fabricate. There is also a need for a method and apparatus for adding one or more additional passbands to an existing radome structure without negatively affecting an already existing passband. In addition, there is a need for a method and apparatus for modifying a material-tuned radome to achieve a desired multi-frequency response without the need for additional dielectric layers.